This invention relates to a process for the preparation of a thermoplastic polymer on the basis of an alkenyl aromatic monomer, an unsaturated dicarboxylic anhydride and, optionally, a rubber.
Copolymers from alkenyl aromatic monomers and unsaturated dicarboxylic anhydrides are known. An example of these copolymers are the copolymers from styrene and maleic anhydride (SMA). Preparation of these copolymers may take place both by batch processes and by continuous processes. The first commercial SMA copolymers were low-molecular products with molecular weights below 5000. These types were applied in, for instance, floor shampoos, floor waxes, emulsion paints and dispersing agents.
Besides these low-molecular products, the high-molecular SMA copolymers are of importance in, for instance, the automotive industry and in domestic articles.
Copolymers of styrene and/or .alpha.-methylstyrene and maleic anhydride are generally prepared by customary methods for, for instance, mass polymerization or solution polymerization. Thus, for instance, according to U.S. Pat. No. 2,971,939 copolymers of styrene and maleic anhydride can be obtained by reacting both monomers, styrene and maleic anhydride, in the presence of a peroxide. Better polymerization control is achieved by application of a solvent, for instance methyl ethyl ketone, acetone, xylene, dioxane, ethylbenzene, dimethylformamide or toluene.
A 50:50 (molar) copolymer can be obtained by discontinuous copolymerization of styrene and maleic anhydride, for instance by precipitation polymerization of styrene and maleic anhydride in aromatic solvents. Copolymers with a lower maleic anhydride content can be obtained if in continuous copolymerization a mixture of much styrene and little maleic anhydride is introduced into a polymerization vessel with vigorous stirring and at a high temperature, while an identical amount of the polymerization mixture is simultaneously removed from the vessel (A. W. Hanson and R. L. Zimmermann, Industrial Engineering Chemistry 49, p. 1803, 1957).
High-molecular SMA polymers can be processed by extrusion and injection moulding. The theoretical upper limit of the MA content is 50 mole % (alternating copolymers), but as high-molecular copolymers these products cannot be processed on account of the high softening point (220.degree. C.). In addition, the brittleness and water sensitivity increase with the MA content.
Copolymers with a lower MA content and a higher molar mass, on the other hand, can be processed much easier. Of great importance in this context is the small dispersion in MA incorporation, which imposes high demands on the homogenity of the reaction mixture. As is evident from U.S. Pat. No. 4,141,934, there already is a large number of publications relating to processes aimed at mixing of styrene and maleic anhydride, without any of these describing a process by which polymers can be prepared without inhomogeneity.
On pages 4 and 5 of European OPI patent application No. 27274, it is stated that the process disclosed in U.S. Pat. No. 4,141,934 does not give the desired result, either, yielding heterogeneous reaction products.
The processes described cannot be used to effect complete polymerization in a one-tank reactor and attain the desired conversion of more than 80% while retaining product homogeneity. To achieve the desired result, a second reactor with metering equipment is used.
During copolymerization of aromatic monomers and unsaturated dicarboxylic anhydrides in mass or in the presence of a solvent, the viscosity of the reaction mixture to be polymerized rises to values that are higher as the degree of conversion is higher, making it even more difficult to obtain a homogeneous mixture.
The object of the invention is to obtain a process for the preparation of a thermoplastic polymer on the basis of an alkenyl aromatic monomer, an unsaturated dicarboxylic anhydride and, optionally, a rubber, that does not exhibit the above disadvantages.